Studies on indicator dispersion in the circulation and exchanges of solutes between blood and tissue are currently focused on the kinetics and mechanisms of exchanges of hydrophilic substrates and ions in the myocardium. A major emphasis is on the simultaneous usage of both tracer extraction techniques and osmotic transient techniques (osmotic weight change and isogravimetric procedures) in isolated perfused heart preparations. Microsphere deposition techniques provides estimates of the heterogeneity of flow which is essential to the interpretation of the data. The validity of mathematical model analysis for describing transcapillary transport mechanisms will be tested by performing tracer and osmotic experiments on mica membranes with one, two, or possibly more families of etched pores of known dimensions. The mathematical formulations describing diffusional and hydrodynamical fluxes through pores and slits are to be developed for the interpretation of the data on myocardial capillaries and the artificial membranes. The results should be of value in defining the mechanisms of exchange in capillaries and in determining the relationships between regional myocardial capillary blood flow and the transport of metabolic substrates in the normal and ischemic heart.